A first terminal is comprised of an amplifier that generally amplifies small signals to large signals in wireless equipment, for example a portable phone, TV, etc. This amplifier is made to have an amplifying operation having low noise and high gain characteristics when the signal is very small. But linearity is demanded rather than the amplifying operation when the signal is relatively large. Therefore, the amplifier satisfies more than two kinds of amplifying modes according to the input signal level, and it is necessary that the amplifier selects one of them in the wireless frequency receiving equipment.
As a low noise amplifier of the prior art, it is disclosed in U.S. Pat. No 6,144,254 that it is possible to switch between a low gain and high gain state.
FIG. 1 shows a circuit diagram of a low noise amplifier disclosed in U.S. Pat. No. 6,144,254.
As shown in FIG. 1, the low noise amplifier comprises a common-emitter BN1 (the first NPN transistor to operate in a high gain state), common-base BN2 (the second NPN transistor to operate in a low gain state), third NPN transistor BN3 (the third NPN transistor for provide bias current in BN2), and resistor R1.
That is, a collector of the first NPN transistor BN1 is connected to an output terminal Pout of an LNA, a base is connected to an input terminal Pin of an LNA and the first bias input terminal Bias1, and an emitter is grounded. The resistor R1 is connected between the first bias input terminal Bias1 and first NPN transistor BN1.
A collector of the second NPN transistor BN2 is connected to the output terminal Pout of the LNA, a base is connected to the second bias input terminal Bias2, and an emitter is connected to the input terminal Pin of LNA and a collector of the third NPN transistor BN3.
A base of BN3 is connected to the third bias input terminal Bias3, and an emitter is grounded.
Hereinafter an operation of the LNA of the prior art is described, referring to FIG. 1.
In the high gain state, the Bias1 is high, and the Bias2 and Bias3 are low. Therefore, the first NPN transistor BN1 is activated and performs the amplifying operation of a high gain, in the high state. Here, the second transistor BN2 and the third transistor BN3 are turned off.
In the low gain state, the Bias2 and Bias3 are high, and the Bias1 is low. Therefore, BN2 and BN3 are activated and perform the low gain amplifying operation, in the low gain state. Here, BN1 is turned off.
The low noise amplifier shown in FIG. 1 selects one of the states between high gain and low gain, and then operates the high-gain or low-gain amplifying operation in accordance with the size of the received signal. But, the circuits operated for each gain state are affected by for the load of the other, because input terminals of the emitter-common first NPN transistor BN1 and base-common second NPN transistor BN2 are directly connected to each other, that is the base of BN1 is directly connected to a emitter of BN2, in the low noise amplifier shown in FIG. 1. Namely, when the low noise amplifier operates in a high gain state, the capacitance of an emitter terminal of BN2 acts as a load for high gain circuits. As a result, the gain, matching, and noise characteristics of the high gain state are not good, and the capability of the low noise amplifier is reduced. In addition, when the low noise amplifier operates in the low gain state, the capability of the low gain state is reduced by the capacitance of the base terminal of BN1 in the same way as with the high gain state. Because two mode impedance levels of the input terminal are at substantially the same level, the impedances act as a load against each other.